This application is a 371 of PCT/EP00/00902 filed Feb. 4, 2000.
The present invention regards mixtures of xcex2-hydroxyalkyl amides and their use as cross-linking agents in the field of powder paints.
The use of xcex2-hydroxyalkyl amides as thermosetting agents in the field of paints is well known, and in particular the use of tetrafunctional xcex2-hydroxyalkyl amides, for example the xcex2-hydroxyalkyl amide of adipic acid of formula (I) given below.
These compounds are mainly used in the preparation of polyester resin-based powder paints, where the resins have free carboxyl groups, a field in which xcex2-hydroxyalkyl amides present remarkable and advantageous cross-linking properties: the thermosetting reaction, which consists in the electrification between the hydroxyl groups of the xcex2-hydroxyalkyl amide and the carboxyl groups of the polyester resin according to the following scheme: 
in fact takes place at a lower temperature than do normal esterification reactions between alcohols and carboxylic acids. Thanks to the activation of the hydroxyl group by the amide group in position xcex2, this reaction takes place already starting from 150xc2x0 C., thus making possible the use of xcex2-hydroxyalkyl amides in the technology for the manufacture of thermosetting powder paints, where the xe2x80x9cbakingxe2x80x9d temperatures of the paints are usually between 140 and 220xc2x0 C.
Against these advantageous characteristics, xcex2-hydroxyalkyl amides, when used as cross-linking agents for polyester polymers with free carboxyl groups, develop water to the extent of approximately 4 mol/mol of cross-linking agent, and this causes surface flaws, such as xe2x80x9cpinholesxe2x80x9d and xe2x80x9ccratersxe2x80x9d in the film of painting product.
The reaction water that forms within the layer of paint in fact develops after the formation of the film, when the electrification reaction takes place. On account of the high temperatures involved, the water comes out in the form of vapour, tearing the film and creating bubbles.
These surface defects appear to a larger extent, the greater the thickness of the film. In this connection it is known that, in the sector of powder paints, in order to obtain films devoid of surface flaws, it is necessary to maintain the thickness of the film below 100 xcexcm by means of a rigorous control of the thicknesses applied in the phase of electrostatic spraying.
This represents a considerable drawback, since a control of this kind is not always possible when shaped items are coated, on which at the pointed parts and edges the rise in the electric field causes larger thicknesses of paint to be deposited.
To limit this phenomenon, degassing additives are generally used, which, by favouring the expulsion of gas and/or water, limit the formation of surface flaws in the film of paint.
The European patent application No. EP-A-322 834 (PPG Ind. Inc.), for example, describes a thermosetting-powder painting composition, based on amorphous polyester resins having free carboxyl groups, which contains tetrafunctional xcex2-hydroxyalkyl amides as cross-linking agents.
After they have been applied on metal substrates, these powder paints are subjected to thermosetting by means of heating in an oven at a temperature between 150 and 200xc2x0 C.
To remove the air that remains entrapped in the film following on the fusion of the powder paint during the thermosetting process, a degassing agent is added, in particular benzoin.
The composition described in EP-A-322 834 in any case does not appear to be suited for the preparation of white paints, light-coloured paints or transparent paints, in that, when undergoing thermosetting, it presents a certain degree of darkening, which increases as the quality of benzoin added, the temperature, and the stoving time increase.
According to EP-A-322 834, the quantity of benzoin required for guaranteeing a good degassing effect ranges between 0.6 and 0.8%, and this causes a marked yellowing of the paint during the xe2x80x9cbakingxe2x80x9d phase.
With lower quantities of benzoin, on the other hand, the critical thickness of the film, i.e., the maximum thickness obtainable without the presence of any surface defects, decreases considerably: a reduction in the quantity of benzoin in 0.2% in order to guarantee a low degree of yellowing of the paint causes the appearance of surface flaws already in a film having a thickness of 80-90 xcexcm.
These attempts thus highlight the impossibility of obtaining a film having a thickness  greater than 100 xcexcm free from surface flaws starting from polyester resin-based compositions, where the resins have free carboxyl groups, comprising xcex2-hydroxyalkyl amides as cross-linking agents, unless degassing additives are added, with consequent increase in the cost and complexity of the preparation process, and also with a consequent worsening, as has already been said, of the optical properties of the paint obtained.
The use of tetrafunctional xcex2-hydroxyalkyl amides alone as cross-linking agents of polyester resins having free carboxyl groups presents another problem, linked to the great difference of polarity between cross-linking agent and polyester resin, which renders difficult the homogenization between the two compounds during preparation of the powder paint.
Since a poor homogenization of the two compounds then results in lower mechanical and aesthetic characteristics of the end product (the surface of the film that has hardened may, for example, be unevenly spread and be affected by the so-called xe2x80x9corange peelxe2x80x9d phenomenon), it is necessary to carry out mixing by melting in an extruder in drastic operating conditions, i.e., at high temperatures and high speeds of the extruder screw, and at times to resort to double extrusion.
An attempt to overcome these drawbacks has been made by DSM NV, which, in the European patent application No. EP-A-652 911, describes the use of N,N-(2-hydroxyethyl)acetamide in combination with a tetrafunctional xcex2-hydroxyalkyl amide as cross-linking agent. Albeit presenting a certain effectiveness in favouring degassing during the cross-linking phase, this compound is very soluble in water and at the temperature of use in liquid. These characteristics render N,N-(2-hydroxyethyl)acetamide practically unusable in the field of powder paints having a base of carboxylated polyester resins: Its incorporation in the mixture of solids that make up the raw material for preparing the powder paint proves in fact particularly problematical and, during storage of the end product, this compound tends to migrate as liquid component, causing sintering of the powder grains.
The problem therefore remains open of having available polyester resin-based thermosetting powder paint compositions, where the resins have free carboxyl groups, containing xcex2-hydroxyalkyl amides as thermosetting agents, which do not require the use of high quantities of degassing additives and do not present the disadvantages described above for the known products, at the same time maintaining good mechanical and aesthetic characteristics.
Now the applicant has found that the use, as cross-linking agent, of a mixture comprising at least two xcex2-hydroxyalkyl amides having a different xcex2-hydroxyalkyl amide functionality in suitable molar ratios presents unexpected advantages in the production of polyester resin-based thermosetting powder paints, where the resins have carboxyl groups.
These cross-linking mixtures have in fact proved effective in favouring the expulsion of the water that forms in the course of the cross-linking process, and have no presented problems of incorporation and homogenization in the steps of processing and storage of powder paints, thus enabling paint films to be obtained that are free from surface flaws at thicknesses of over 130 xcexcm, and moreover have good mechanical characteristics and good characteristics of gloss, adherence and resistance to atmospheric agents.
The subject of the present invention is therefore a cross-linking mixture for polymers having free carboxyl groups which is suitable for the preparation of powder paints and comprises at least two compounds having xcex2-hydroxyalkyl amide groups, characterized in that it has a mean xcex2-hydroxyalkyl amide functionality of between 2.1 and 3.9.
The powder paint containing the said cross-linking mixture and the process for its preparation constitute further subjects of the present invention.
The characteristics and advantages of the xcex2-hydroxyalkyl amide mixtures as cross-linking agents according to the present invention will be illustrated in detail in the ensuing description.